This program encompasses a group endeavor between physicians, biomedical engineers and mathematicians in a collaborative effort to investigate the electrophysiological and mechanical function of the heart in a variety of conditions in children and in some adults. The electrophysiology studies concentrate on the investigation of extracellular-intracellular potential relationships of both excitation and repolarization, the development of techniques to elucidate the mechanisms of arrhythmias (reentry vs. repetitive firing), the relationship between epicardial potential distributions and those on the body surface, the potential distribution intramurally in the ventricles, and the use of isopotential maps in human and animal studies to ultimately obtain a solution of the forward and inverse electrocardiographic problem. One major emphasis currently is on the use of a new mathematical approach to compute the body surface potential distributions from the potential distribution surrounding the heart and to test this solution with measured data in intact animals. A major emphasis in the extracellular studies is to achieve a better understanding of ventricular repolarization, as studied through analysis of ectopic beats, to determine the genesis of the T wave. The cardiac contractility studies concentrate on developing a way of evaluating contractility which satisfies all the criteria of an index of cardiac contractility--independence of muscle fiber length, sensitivity to positive and negative inotropic agents, and independence of drug response to muscle fiber length. These studies primarily involve analysis of short-term force-frequency relationships. These relationships have been studied primarily in vitro and currently are being extended to the intact conscious dog with plans to ultimately study these relationships in patients. Other studies include the evaluation of a recently developed technique for obtaining and recording light diffraction patterns of sarcomeres in living preparations of cardiac muscle through the use of laser light diffraction methods. Heat jump techniques are being utilized to study the relationship between tension and time and the mediating influence of the shape of the action potential.